This invention relates to thyristors of the type including field effect transistors (FETs) for both turning on and turning off the thyristors, and particularly to an improved thyristor having turn-on FETs of increased capacity for more rapidly turning on the thyristors.
FET controlled thyristors, also known as MCTs (Metal-oxide-semiconductor Controlled Thyristors), are now generally known and described, for example, in U.S. Pat. Nos. 4,816,892 and 4,857,977, the subject matter of which is incorporated herein by reference. In such thyristors, turn off is accomplished by diverting current from one of the base regions around (rather than through) the p-n junction formed by the base region and its adjacent emitter region, thereby turning off injection of charge carriers from the emitter into the base. Because generally large current densities are involved, the diverting path length must be relatively short, to prevent high voltage drops therealong, and the thyristors are thus made of a plurality of small cells, each cell being small enough to provide a short current diverting path. The cells are connected in parallel to provide high power capacity in the composite thyristor.
The mechanism for turning off each cell comprises a field effect transistor disposed within each cell at a surface thereof, the channel region of the FETs serving as low resistance switches for creating the current diverting path.
Turning on of such thyristors is accomplished by basically short circuiting a portion of one of the blocking p-n junctions of the thyristor and injecting current from one of the thyristor main terminals directly into one of the base regions for forward biasing the emitter region adjacent thereto. The mechanism for turning on the thyristor also comprises FETs at the substrate surface, the channels of which selectively provide low resistance paths through the blocking p-n junctions.
In order to turn off the thyristor, a relatively large proportion of the current flowing between the device main terminals must be diverted through the channels of the turn-off FETs. Obviously, the channels must be sufficiently low in impedance to carry the current. Conversely, in order to turn on a thyristor, only relatively small amounts of current must be injected into the base region. This is because, once a small portion of an emitter p-n junction is forward biased to inject current into the base region, the process becomes self-sustaining and the initially small turned-on portion of each cell rapidly expands to fully turn on all other portions of the cell and then to neighboring cells until the entire device is fully on.
Because the channels of the turn-on FETs can be smaller than the channels of the turn-off FETs, one practice in the past is to arrange the cells in groups of 9 contiguous cells, i.e., a 3.times.3 block of cells including a central cell surrounded by 8 cells. This is illustrated in FIG. 1. Each peripheral cell includes a turn-off FET, but only the central cell includes a turn-on FET which is effective for turning on each of the surrounding cells. To avoid reducing the area available for the turn-on FET, the central cell does not include a turn-off FET.
One shortcoming of this arrangement is that the composite thyristor is not of uniform structure or characteristics throughout. This leads to current density variations and problems related thereto.
Another shortcoming of this arrangement is that main terminal current also flows through the central cells when the composite thyristor is in its on or conductive state, and turning off the thyristor also requires turning off the central cells. Because these cells do not have turn-off FETs, however, the currents through the central cells must be diverted to the turn-off FETs of the surrounding cells. This tends to increase the turn-off time and reduce the turn-off capability of the composite thyristor. To counter this effect, one practice is to design at least some of the cells surrounding the central cells with lower main terminal current carrying capacity, thereby reducing the current flow through these surrounding cells and reducing the amount of current that must be diverted therefrom. However, this reduces the overall current capacity of the composite thyristor and is wasteful of space.
In some instances, it is desirable to increase the turn-on speed of the composite thyristor. This can be accomplished by increasing the size or number of the central cells at the expense of the size or number of the surrounding cells, but this causes an undesirable increase in the turn-off time of the thyristor as well as a further reduction in the current carrying capacity of the composite thyristor.